Trouble Graphing an Equation

[legallady]

Hi,

I am having trouble graphing an equation, and I can't figure out what about the equation is preventing me from doing so. The equation is:

8(x+1)-(xy)=z [or 8(x+1)-(xy)-z=0], where x & y are greater than 7, and z is greater than 0.

Any thoughts?? (I'm sure there is an obvious answer I am overlooking).

Thanks!!!

 

[tkhunny]

What have you tried? What tools have you available?

 

[legallady]

tkhunny,

As you may have gleaned from my username, I am not especially math inclined. I'm more of a word person. Either way, I've tried to use a couple graphing programs online, and that didn't work (I kept getting error messages). Then I tried sketching a 3D graph myself, but once there are more than a few points, it's difficult to see the field that develops on a 2D piece of paper (that, and I'm not even sure I'm drawing it properly).

Any suggestions? Does this look like an equation that can be graphed??

Thank you.

 

[galactus]

Since z > 0, you could set z = 0 and solve for y. This would then give you an idea of values that
would result in z being greater than 0. Since x and y have to be > 7, plug in x=8 and get y=9

So, there's a point to plot: (8,9,0). And so on.

\(\displaystyle y=\frac{8(x+1)}{x}\). Now, plug in values for x and get the corresponding y.

Here is a half-a**ed graph. See all of that extending down into negative territory where z<0?

 

[legallady]

Galactus,

Wow, that graph is great, thank you. What program did you use to make that??

Yes, I see the area where z is negative...

(While the positive area is more relevant to my project, it's actually still helpful to see that negative area)

 

[galactus]

I used Maple to generate that graph. No, it is not free.

Now, you have me wondering. Is there a freeware 3D graphing utility out there somewhere?.

Here is a link to a very nice graphing utility that is FREE. It is not 3D, though.

www.padowan.dk